Is this a good place for questions on technical writing, specifically, writing on software development matters, or is there a more specialised StackExchange site for that?
I believe that technical writing should be on topic here.
To address what Dori said: The reason I see for no non-fiction or technical writing questions is just because no one has asked them. I fully support adding those to our site definition. Just because they may be rarer does not mean that they should be discouraged.
If there is a Technical Writing proposition, it should be closed as a duplicate of this site. We don't have separate sites for C#, PHP, Python, etc. We have StackOverflow. Not everyone on StackOverflow is a master of every language, but there are experts from all realms. The same would be true here. Not everyone would participate in technical writing questions, but the people who are interested in it will.
Even all that aside, even if a technical writing page does open, we cannot allow our definition to be defined by what other resources exist on the Internet, even from within our own network. If Technical Writing does somehow manage to survive and get its own public release despite this site's existence, we should still allow Technical Writing questions.